1. Field of the Invention
The present invention relates to a digital camera system in which lenses are interchangeable, and an intermediate adapter for use in the digital camera system.
2. Description of the Related Art
Heretofore, in a camera system in which lenses are interchangeable, a distance (flange focal length) from a mount surface of the interchangeable lens to an image plane is defined, and the interchangeable lens attachable to the camera is designed based on the defined flange focal length. Therefore, in a case where the interchangeable lens designed so as to be applied to a camera having a long flange focal length is attached to another camera having a short flange focal length, the flange focal length needs to be adjusted using an intermediate adapter or the like. As to this type of intermediate adapter, a large number of proposals are described in Japanese Patent Application Laid-Open No. 2005-70711.
In addition, in a conventional digital camera, an optical low-pass filter (hereinafter referred to as the optical LPF) made of crystals or the like having birefringent characteristics is disposed on the front of an image sensor in order to reduce false color and the like derived from the high-frequency components of a photographed image. This optical LPF splits an incident luminous flux into normal light which is not double refracted and abnormal light (birefringent light), and the split birefringent light is struck on pixels disposed adjacent to each other in the image sensor. Therefore, to prevent generation of Moire fringes, a thickness of the optical LPF has to be appropriately set in accordance with a pixel pitch.
Moreover, between a photographing lens and the image sensor, in addition to the optical LPF, flat-plate optical elements are installed such as a dustproof filter, infrared ray absorbing glass and protective glass for storing the image sensor in a sealed state. These optical elements have peculiar optical characteristics (e.g., a refractive index, etc.), respectively. Therefore, in the digital camera in which the lenses are interchangeable, unless the interchangeable lens is designed in consideration of not only a mechanical distance (flange focal length) from the mount surface of the interchangeable lens to the image plane but also a change of an optical distance (optical path length) attributable to the plurality of optical elements, a generally high image quality of the digital camera system cannot be obtained. This also applies to the above intermediate adapter.
However, in a conventional adapter for adjusting the flange focal length as described in Japanese Patent Application Laid-Open No. 2005-70711, only adjustment of the flange focal length is noted. In Japanese Patent Application Laid-Open No. 2005-70711, there is not any idea that the optical path length is adapted.